List the functions that skeletal muscles perform. The structure of skeletal muscle and its properties

It forms the skeletal muscles of humans and animals, designed to perform various actions: body movements, contraction of the vocal cords, breathing. Muscles are made up of 70-75% water.

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The structure of the muscle cell

The structure of skeletal striated muscles

Contraction of muscle fibers

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We examined the mechanism of muscle contraction at the molecular level. Now let's talk about the structure of the muscle itself and how it is connected with the surrounding tissues. I'll draw the biceps. Like this... Contracting biceps... Here is the elbow, here is the hand. Here is such a person's biceps during contraction. Probably, you have all seen drawings of muscles, at least schematically, the muscle is attached to the bones on both sides. I'll mark the bones. Schematically ... The muscle on both sides is attached to the bone with the help of tendons. Here we have a bone. And here too. And with white color I will designate the tendons. They attach muscles to bones. And this is a tendon. The muscle is attached to two bones; when contracted, it moves part of the skeletal system. Today we are talking about skeletal muscles. Skeletal… Other types include smooth muscle and cardiac muscle. Cardiac muscles, as you understand, are in our heart; and smooth muscles contract involuntarily and slowly, they form, for example, the digestive tract. I will make a video about them. But in most cases, the word “muscles” refers to the skeletal muscles that move the bones and make it possible to walk, talk, chew, and the like. Let's look at these muscles in more detail. If you look at the biceps muscle in cross section... cross section of the muscle... I'll make the drawing bigger. Let's draw the biceps... No, let it be just an abstract muscle. Let's take a look at it in cross section. Now we will find out what the muscle has inside. The muscle passes into the tendon. Here is the tendon. And the muscle has a shell. There is no clear boundary between the shell and the tendon; the sheath of the muscle is called the epimysium. This is connective tissue. It surrounds the muscle, performs some protective functions, reduces the friction of the muscle on the bone and other tissues, in our example, the tissue of the hand. There is also connective tissue inside the muscle. I'll take another color. Orange. This is a connective tissue sheath; it surrounds bundles of muscle fibers of different thicknesses. It's called the perimysium, it's the connective tissue inside the muscle. Perimisium... And each of these bundles is surrounded by perimysium... If we consider it in more detail... Here is one such bundle of muscle fibers surrounded by perimysium... Let's take this bundle. It is surrounded by a sheath called the perimysium. This is such a "smart" word for connective tissue. There, of course, there are other tissues - nerve fibers, capillaries, because blood and nerve impulses need to be supplied to the muscle. So there, in addition to the connective tissue, there are other tissues that ensure the life of muscle cells. Each of these groups of fibers - and these are large groups of muscle fibers - is called a bundle. It's a bundle... A bundle. There is also connective tissue inside such a bundle; it is called endomysium. Now I will mark it. Endomysium. I repeat: in the composition of the connective tissue there are nerve fibers, capillaries - everything necessary to ensure contact with muscle cells. We are looking at the structure of the muscle. This is the endomysium. The green color indicates the connective tissue, which is called the endomysium. Endomysium. But such a “fiber”, surrounded by endomysium, is a muscle cell. Muscle cell. I'll mark it with a different color. Here is such an elongated cage. I'll "pull" it out a bit. Muscle cell. Let's look inside it and see how myosin and actin filaments are located there. So, here is a muscle cell or muscle fiber. Muscle fiber… You will often see two prefixes; the first is "myo", derived from the Greek word for "muscle"; And the second is “sarco”, for example, in the words “sarcolemma”, “sarcoplasmic network”, which comes from the Greek word “meat”, “flesh”. It was preserved in a number of words, for example, "sarcophagus". "Sarco" means flesh, "myo" means muscle. So, here is the muscle fiber. Or a muscle cell. Let's look at it in more detail. Now I'll draw it bigger. Muscle cell, otherwise called muscle fiber. "Fiber" - because it is much longer in length than in width; it has an elongated shape. Now I will draw. Here is my muscle cell ... Let's consider it in cross section. Muscle fiber ... They are relatively short - a few hundred micrometers - and very long, at least by cellular standards. Let's have a few centimeters. Imagine such a cage! It is very long, so it has several cores. And to indicate the nuclei, I'll tweak my drawing. I will add such tubercles on the cell membrane - under them there will be nuclei. Let me remind you that this is just one muscle cell; such cells are very long, so they have several nuclei. Here is the cross section. As I said, there are several nuclei in a cell. Imagine that the membrane is transparent; here is one core, here is another, here is the third, and the fourth. Many nuclei are needed in order not to waste time on overcoming long distances by proteins; let's say from this nucleus to this part of the cell. In a multinucleated cell, DNA information is always nearby. If I'm not mistaken, there are an average of thirty cores in one millimeter of muscle tissue. I don’t know how many nuclei are in our cell, but they are located directly under the membrane - and you remember what it is called from the last lesson. The membrane of the muscle cell is called the sarcolemma. Let's write down. Sarcolemma. Emphasis on the third syllable. Here are the cores. The nucleus... And if we look at the cross section, we will see even finer structures, they are called myofibrils. These are the thread-like structures inside the cell. I will draw one of them in the picture. Here is one of those threads. This is a myofibril. Myofibril... If you look at it through a microscope, you can see the grooves. These are the grooves... Here, here and here... And a couple of thin ones... Inside the myofibrils, the interaction of myosin and actin filaments takes place. Let's zoom in even more. So we will increase until we reach the molecular level. So, myofibril; it is located inside the muscle cell or muscle fiber. A muscle fiber is a muscle cell. A myofibril is a filamentous structure within a muscle cell. It is myofibrils that provide muscle contraction. I will draw the myofibril on a larger scale. Something like this... There are stripes on it... This is called striation. Narrow stripes. More ... There are wider stripes. I'll try to draw as accurately as possible. Here is another strip here ... And then everything repeats. Each of these repeating regions is called a sarcomere. This is a sarcomere. Sarcomere... Such areas are located between the so-called Z-lines. The terms were coined when researchers first saw these lines under a microscope. We will talk about how they are related to myosin and actin very soon. This zone is usually called Disk A or A-disk. But this zone here and here - disk I or I-disk. In a couple of minutes, we will find out how they are related to the mechanisms, molecules, which we talked about in the last lesson. If we look inside the myofibrils, we make a cross section of it, we divide it into sections parallel to the screen we look at, that's what we see. So, here's one Z-line. Z-line… Next Z-line. I draw one sarcomere on a large scale. Neighboring Z-line. And here we go to the molecular level, as I promised. Here are the actin filaments. I'll mark them with wavy lines. Let there be three ... I will sign them ... Actin filaments ... And between the actin filaments - myosin. I'll draw them in a different color ... Remember, there are two heads on myosin fibers. Each of them has two heads that slide or "crawl" along the actin fibers. I'll name a few... Here they are attached... Now we'll see what happens when a muscle contracts. Let's draw more myosin fibers. In fact, there are incomparably more myosin heads, but we have a schematic drawing. These are the filaments of the myosin protein, they are twisted, as we saw in the last lesson; here is another one. I will outline it schematically ... You can immediately notice that the myosin filaments are in the A-disk. This is the area of ​​the A-disk. The A-disk... Sections of the actin and myosin filaments overlap each other, but the I-disk is the area where there is no myosin, only actin. I-disc... Myosin filaments are held by titin; it is a resilient, elastic protein. I will color it in a different color. These spirals... Myosin filaments are held by titin. It connects myosin to the Z-zone. So what's going on? When a neuron is excited... Let's draw the terminal branch of the neuron, more precisely, the terminal branch of the axon. This is a motor neuron. He gives the command to the myofibril to contract. The action potential propagates along the membrane in all directions. And in the membrane, we remember, there are T-tubules. The action potential passes through them into the cell and continues to spread. The sarcoplasmic reticulum releases calcium ions. Calcium ions bind to troponin, which attaches to actin filaments, tropomyosin shifts, and myosin can interact with actin. Myosin heads can use the energy of ATP and glide along actin filaments. Remember this "workflow"? This can be thought of as a movement of the actin filaments to the right (away from us) or as a movement of the myosin head to the left (away from us); It's a mirror movement, right? Look, the myosin stays in place, and the actin filaments are attracted to each other. To each other. This is how the muscle contracts. So, we have gone from the general view of the muscle to the processes occurring at the molecular level, which we talked about in previous lessons. These processes occur in all myofibrils inside the cell, because the sarcoplasmic reticulum releases calcium into the cytoplasm, another name for which is myoplasm, because we are talking about a muscle cell, the whole cell. Calcium enters all myofibrils. There is enough calcium ion to bind to all - or most - of the troponin proteins on the actin filaments, and the entire muscle contracts. Individual muscle fibers, muscle cells, probably have a small contractile force. By the way, when one or more fibers contract, you feel twitches. But when they are all working, their strength is enough to do the work, move our bones, lift weights. I hope the lesson was useful.

Histogenesis

The source of development of skeletal muscles are myotome cells - myoblasts. Some of them are differentiated in the places of formation of the so-called autochthonous muscles. Others migrate from myotomes to mesenchyme; at the same time, they are already determined, although outwardly they do not differ from other cells of the mesenchyme. Their differentiation continues in the places of laying of other muscles of the body. In the course of differentiation, 2 cell lines arise. The cells of the first merge, forming symplasts - muscle tubes (myotubes). The cells of the second group remain independent and differentiate into myosatellites (myosatellitocytes).

In the first group, differentiation of specific organelles of myofibrils occurs, gradually they occupy most of the lumen of the myotube, pushing the nuclei cells to the periphery.

The cells of the second group remain independent and are located on the surface of the myotubes.

Structure

The structural unit of muscle tissue is the muscle fiber. It consists of myosymplast and myosatellitocytes (satellite cells) covered by a common basement membrane. The length of the muscle fiber can reach several centimeters with a thickness of 50-100 micrometers.

Skeletal muscles are attached to bones or to each other by strong, flexible tendons.

The structure of the myosymplast

Myosymplast is a collection of fused cells. It has a large number of nuclei located along the periphery of the muscle fiber (their number can reach tens of thousands). Like the nuclei, on the periphery of the symplast there are other organelles necessary for the functioning of the muscle cell - the endoplasmic reticulum (sarcoplasmic reticulum), mitochondria, etc. The central part of the symplast is occupied by myofibrils. The structural unit of the myofibril is the sarcomere. It consists of actin and myosin molecules, it is their interaction that provides a change in the length of the muscle fiber and, as a result, muscle contraction. The composition of the sarcomere also includes many auxiliary proteins - titin, troponin, tropomyosin and other motor neurons. The number of muscle fibers that make up one IU varies in different muscles. For example, where fine control of movements is required (in the fingers or in the muscles of the eye), the motor units are small, containing no more than 30 fibers. And in the calf muscle, where fine control is not needed, there are more than 1000 muscle fibers in the IU.

The motor units of one muscle can be different. Depending on the speed of contraction, motor units are divided into slow (slow (S-ME)) and fast (fast (F-ME)). And F-ME, in turn, is divided according to resistance to fatigue into fast-fatigue-resistant (FR-ME)) and fast-fatigue (fast-fatigable (FF-ME)).

The ME motor neurons innervating these data are subdivided accordingly. There are S-motor neurons (S-MN), FF-motor neurons (F-MN) and FR-motoneurons (FR-MN) S-ME are characterized by a high content of myoglobin protein, which is able to bind oxygen (O2). Muscles predominantly composed of this type of ME are called red because of their dark red color. Red muscles perform the function of maintaining a person's posture. The ultimate fatigue of such muscles occurs very slowly, and the restoration of functions occurs, on the contrary, very quickly.

This ability is due to the presence of myoglobin and a large number of mitochondria. Red muscle IUs tend to contain large amounts of muscle fibers. FR-MEs are muscles that can perform fast contractions without noticeable fatigue. FR-ME fibers contain a large number of mitochondria and are able to form ATP through oxidative phosphorylation.

As a rule, the number of fibers in FR-ME is less than in S-ME. FF-ME fibers are characterized by a lower content of mitochondria than in FR-ME, and also by the fact that ATP is formed in them due to glycolysis. They lack myoglobin, which is why muscles composed of this type of ME are called white. White muscles develop a strong and rapid contraction, but tire rather quickly.

Function

This type of muscle tissue provides the ability to perform voluntary movements. A contracting muscle acts on the bones or skin to which it attaches. In this case, one of the points of attachment remains motionless - the so-called fixation point(lat. púnctum fíxsum), which in most cases is considered as the initial section of the muscle. The moving piece of muscle is called moving point, (lat. púnctum móbile), which is the place of its attachment. However, depending on the function performed, punctum fixum can act as punctum mobile, and vice versa.

Muscle tissue is recognized as the dominant tissue of the human body, the share of which in the total weight of a person is up to 45% in men and up to 30% in the fair sex. Musculature includes a variety of muscles. There are more than six hundred types of muscles.

The importance of muscles in the body

Muscles play an extremely important role in any living organism. With their help, the musculoskeletal system is set in motion. Thanks to the work of muscles, a person, like other living organisms, can not only walk, stand, run, make any movement, but also breathe, chew and process food, and even the most important organ - the heart - also consists of muscle tissue.

How are muscles worked?

The functioning of muscles occurs due to the following properties:

• Excitability is an activation process manifested as a response to a stimulus (usually an external factor). The property manifests itself in the form of a change in the metabolism in the muscle and its membrane.
• Conductivity is a property that means the ability of muscle tissue to transmit a nerve impulse formed as a result of exposure to an irritant from a muscle organ to the spinal cord and brain, as well as in the opposite direction.
• Contractility - the final action of the muscles in response to a stimulating factor, manifests itself in the form of shortening of the muscle fiber, the tone of the muscles also changes, that is, the degree of their tension. At the same time, the rate of contraction and the maximum tension of the muscles can be different as a result of the different influence of the stimulus.

It should be noted that muscle work is possible due to the alternation of the above properties, most often in the following order: excitability-conductivity-contractility. If we are talking about voluntary work of the muscles and the impulse comes from the central nervous system, then the algorithm will look like conduction-excitability-contractility.

Muscle structure

Any human muscle consists of a set of oblong cells acting in the same direction, called a muscle bundle. The bundles, in turn, contain muscle cells up to 20 cm long, also called fibers. The shape of the cells of the striated muscles is oblong, smooth - fusiform.

A muscle fiber is an elongated cell bounded by an outer shell. Under the shell, parallel to each other, protein fibers capable of contracting are located: actin (light and thin) and myosin (dark, thick). In the peripheral part of the cell (near the striated muscles) there are several nuclei. Smooth muscles have only one nucleus, it is located in the center of the cell.

Classification of muscles according to various criteria

The presence of various characteristics that are different for certain muscles allows them to be conditionally grouped according to a unifying feature. To date, anatomy does not have a single classification by which human muscles could be grouped. Muscle types, however, can be classified according to various criteria, namely:

1. In shape and length.
2. According to the functions performed.
3. In relation to the joints.
4. By localization in the body.
5. By belonging to certain parts of the body.
6. According to the location of the muscle bundles.

Along with the types of muscles, three main muscle groups are distinguished depending on the physiological features of the structure:

1. Striated skeletal muscles.
2. Smooth muscles that make up the structure of internal organs and blood vessels.
3. heart fibres.

The same muscle can simultaneously belong to several groups and types listed above, since it can contain several cross-signs at once: shape, functions, relation to a body part, etc.

Shape and size of muscle bundles

Despite the relatively similar structure of all muscle fibers, they can be of different sizes and shapes. Thus, the classification of muscles according to this feature distinguishes:

1. Short muscles move small parts of the human musculoskeletal system and, as a rule, are located in the deep layers of the muscles. An example is the intervertebral spinal muscles.
2. Long, on the contrary, are localized on those parts of the body that make large amplitudes of movement, such as limbs (arms, legs).
3. Wide ones cover mainly the torso (on the stomach, back, sternum). They can have different directions of muscle fibers, thereby providing a variety of contractile movements.

Various forms of muscles are also found in the human body: round (sphincters), straight, square, rhomboid, spindle-shaped, trapezoid, deltoid, serrated, one- and two-pinnate and muscle fibers of other shapes.

Varieties of muscles according to their functions

Human skeletal muscles can perform various functions: flexion, extension, adduction, abduction, rotation. Based on this feature, the muscles can be conditionally grouped as follows:

1. Extensors.
2. Flexors.
3. Leading.
4. Discharging.
5. Rotational.

The first two groups are always on the same part of the body, but on opposite sides in such a way that when the first contract, the second relax, and vice versa. The flexor and extensor muscles move the limbs and are antagonist muscles. For example, the biceps brachii muscle flexes the arm, while the triceps extends it. If, as a result of the work of the muscles, a part of the body or an organ moves towards the body, these muscles are adductors, if in the opposite direction, they are abducting. The rotators provide circular movements of the neck, lower back, head, while the rotators are divided into two subspecies: pronators, which move inward, and arch supports, which provide movement to the outside.

In relation to the joints

The musculature is attached with the help of tendons to the joints, setting them in motion. Depending on the attachment option and the number of joints that the muscles act on, they are: single-joint and multi-joint. Thus, if the musculature is attached to only one joint, then it is a single-joint muscle, if to two, it is bi-articular, and if there are more joints, it is multi-joint (flexors / extensors of the fingers).

As a rule, single-articular muscle bundles are longer than multi-articular ones. They provide a fuller range of motion of the joint relative to its axis, since they spend their contractility on only one joint, while polyarticular muscles distribute their contractility over two joints. The latter types of muscles are shorter and can provide much less mobility while simultaneously moving the joints to which they are attached. Another property of multi-joint muscles is called passive insufficiency. It can be observed when, under the influence of external factors, the muscle is completely stretched, after which it does not continue to move, but, on the contrary, slows down.

Localization of muscles

Muscle bundles can be located in the subcutaneous layer, forming superficial muscle groups, and maybe in deeper layers - these include deep muscle fibers. For example, the musculature of the neck consists of superficial and deep fibers, some of which are responsible for the movements of the cervical region, while others pull the skin of the neck, the adjacent area of ​​the skin of the chest, and also participate in turning and tipping the head. Depending on the location in relation to a particular organ, there can be internal and external muscles (external and internal muscles of the neck, abdomen).

Types of muscles by body parts

In relation to parts of the body, the muscles are divided into the following types:

1. The muscles of the head are divided into two groups: chewing, responsible for the mechanical grinding of food, and facial muscles - types of muscles, through which a person expresses his emotions, mood.
2. The muscles of the body are divided into anatomical sections: cervical, pectoral (large sternal, trapezius, sternoclavicular), dorsal (rhomboid, latissimus dorsalis, large round), abdominal (internal and external abdominal, including the press and diaphragm).
3. Muscles of the upper and lower extremities: shoulder (deltoid, triceps, biceps brachialis), elbow flexors and extensors, gastrocnemius (soleus), tibia, foot muscles.

Varieties of muscles according to the location of muscle bundles

Muscle anatomy in different species may differ in the location of muscle bundles. In this regard, muscle fibers such as:

1. Cirrus resemble the structure of a bird's feather, in which the muscle bundles are attached to the tendons on only one side, and the other diverge. The pinnate form of the arrangement of muscle bundles is characteristic of the so-called strong muscles. The place of their attachment to the periosteum is quite extensive. As a rule, they are short and can develop great strength and endurance, while muscle tone will not be very large.
2. Muscles with parallel arrangement of bundles are also called dexterous. Compared to feathery, they are longer, while less hardy, but they can perform more delicate work. When reduced, the voltage in them increases significantly, which significantly reduces their endurance.

Muscle groups by structural features

Accumulations of muscle fibers form whole tissues, the structural features of which determine their conditional division into three groups:

Skeletal muscles - the active part of the musculoskeletal system, which also includes bones, ligaments, tendons and their joints. From a functional point of view, motoneurons that cause excitation of muscle fibers can also be attributed to the motor apparatus. The axon of the motor neuron branches at the entrance to the skeletal muscle, and each branch is involved in the formation of a neuromuscular synapse on a separate muscle fiber.

The motor neuron, together with the muscle fibers it innervates, is called the neuromotor (or motor) unit (MU). In the eye muscles, one motor unit contains 13-20 muscle fibers, in the muscles of the body - from 1 ton of fibers, in the soleus muscle - 1500-2500 fibers. Muscle fibers of one MU have the same morphofunctional properties.

skeletal muscle functions are: 1) the movement of the body in space; 2) movement of body parts relative to each other, including the implementation of respiratory movements that provide ventilation of the lungs; 3) maintaining the position and posture of the body. In addition, striated muscles are important in generating heat to maintain temperature homeostasis and in storing certain nutrients.

Physiological properties of skeletal muscles allocate:

1)excitability. Due to the high polarization of the membranes of striated muscle fibers (90 mV), their excitability is lower than that of nerve fibers. Their action potential amplitude (130 mV) is greater than that of other excitable cells. This makes it quite easy to record the bioelectrical activity of skeletal muscles in practice. The duration of the action potential is 3-5 ms. This determines the short period of absolute refractoriness of muscle fibers;

conductivity. The speed of excitation along the membrane of the muscle fiber is 3-5 m/s;

contractility. Represents a specific property of muscle fibers to change their length and tension during the development of excitation.

Skeletal muscles also have elasticity and viscosity.

Modes and types of muscle contractions. Isotonic mode - the muscle shortens in the absence of an increase in its tension. Such a contraction is possible only for an isolated (removed from the body) muscle.

Isometric mode - muscle tension increases, and the length practically does not decrease. Such a reduction is observed when trying to lift an unbearable load.

Auxotonic Mode the muscle shortens and its tension increases. Such a reduction is most often observed in the implementation of human labor activity. Instead of the term "auxotonic mode", the name is often used concentric mode.

There are two types of muscle contractions: single and tetanic.

single muscle contraction manifests itself as a result of the development of a single wave of excitation in muscle fibers. This can be achieved by exposing the muscle to a very short (about 1 ms) stimulus. In the development of a single muscle contraction, a latent period, a shortening phase and a relaxation phase are distinguished. Muscle contraction begins to manifest itself after 10 ms from the onset of exposure to the stimulus. This time interval is called the latent period (Fig. 5.1). This will be followed by the development of shortening (duration about 50 ms) and relaxation (50-60 ms). It is believed that the entire cycle of a single muscle contraction takes an average of 0.1 s. But it should be borne in mind that the duration of a single contraction in different muscles can vary greatly. It also depends on the functional state of the muscle. The rate of contraction and especially relaxation slows down with the development of muscle fatigue. Fast muscles that have a short period of single contraction include the muscles of the tongue and the closing eyelid.

Rice. 5.1. Time ratios of different manifestations of skeletal muscle fiber excitation: a - ratio of the action potential, release of Ca 2+ into the sarcoplasm and contraction: / - latent period; 2 - shortening; 3 - relaxation; b - the ratio of action potential, contraction and level of excitability

Under the influence of a single stimulus, an action potential first arises and only then a shortening period begins to develop. It continues even after the end of repolarization. The restoration of the original polarization of the sarcolemma also indicates the restoration of excitability. Consequently, against the background of developing contraction in muscle fibers, new waves of excitation can be induced, the contractile effect of which will be summed up.

tetanic contraction or tetanus called muscle contraction, which appears as a result of the occurrence in motor units of numerous waves of excitation, the contractile effect of which is summarized in amplitude and time.

There are dentate and smooth tetanus. To obtain a dentate tetanus, it is necessary to stimulate the muscle with such a frequency that each subsequent impact is applied after the shortening phase, but until the end of relaxation. Smooth tetanus is obtained with more frequent stimulations, when subsequent exposures are applied during the development of shortening of the muscle. For example, if the shortening phase of a muscle is 50 ms, and the relaxation phase is 60 ms, then to obtain a dentate tetanus, it is necessary to stimulate this muscle with a frequency of 9-19 Hz, to obtain a smooth one - with a frequency of at least 20 Hz.

Despite

Pessimum

200 Hz

50 Hz

Rice. 5.2. Dependence of the amplitude of contraction on the frequency of stimulation (strength and duration of stimuli are unchanged)

To demonstrate various types of tetanus, the registration of contractions of an isolated frog gastrocnemius muscle on a kymograph is usually used. An example of such a kymogram is shown in Fig. 5.2. The amplitude of a single contraction is minimal, increases with serrated tetanus, and becomes maximum with smooth tetanus. One of the reasons for this increase in amplitude is that when frequent waves of excitation occur in the sarcoplasm of muscle fibers, Ca 2+ accumulates, stimulating the interaction of contractile proteins.

With a gradual increase in the frequency of stimulation, the increase in strength and amplitude of muscle contraction goes only up to a certain limit - optimum response. The frequency of stimulation that causes the greatest response of the muscle is called optimal. A further increase in the frequency of stimulation is accompanied by a decrease in the amplitude and strength of contraction. This phenomenon is called pessimum response, and the frequencies of irritation exceeding the optimal value are pessimal. The phenomena of optimum and pessimum were discovered by N.E. Vvedensky.

When evaluating the functional activity of muscles, they talk about their tone and phasic contractions. muscle tone called a state of continuous continuous tension. In this case, there may be no visible shortening of the muscle due to the fact that excitation does not occur in all, but only in some motor units of the muscle, and they are not excited synchronously. phasic muscle contraction called short-term shortening of the muscle, followed by its relaxation.

Structurally- functional characteristics of the muscle fiber. The structural and functional unit of the skeletal muscle is the muscle fiber, which is an elongated (0.5-40 cm long) multinucleated cell. The thickness of muscle fibers is 10-100 microns. Their diameter can increase with intense training loads, while the number of muscle fibers can increase only up to 3-4 months of age.

The muscle fiber membrane is called sarcolemma cytoplasm - sarcoplasm. In the sarcoplasm there are nuclei, numerous organelles, the sarcoplasmic reticulum, which includes longitudinal tubules and their thickenings - tanks, which contain reserves of Ca 2+. Tanks are adjacent to transverse tubules penetrating the fiber in the transverse direction (Fig. 5.3).

In the sarcoplasm, about 2000 myofibrils (about 1 micron thick) run along the muscle fiber, which include filaments formed by the plexus of contractile protein molecules: actin and myosin. Actin molecules form thin filaments (myofilaments) that lie parallel to each other and penetrate a kind of membrane called the Z-line or stripe. Z-lines are located perpendicular to the long axis of the myofibril and divide the myofibril into sections 2–3 µm long. These areas are called sarcomeres.

Sarcolemma Cistern

transverse tubule

Sarcomere

Jj3H ssss s_ z zzzz tccc ;

; zzzz ssss

zzzzz ssss

j3333 CCCC£

J3333 c c c c c_

Sarcomere shortened

Sarcomere relaxed

Rice. 5.3. The structure of the muscle fiber sarcomere: Z-lines - limit the sarcomere, /! - anisotropic (dark) disk, / - isotropic (light) disk, H - zone (less dark)

The sarcomere is the contractile unit of the myofibril. In the center of the sarcomere, thick filaments formed by myosin molecules lie strictly ordered one above the other, and thin filaments of actin are similarly located along the edges of the sarcomere. The ends of the actin filaments extend between the ends of the myosin filaments.

The central part of the sarcomere (width 1.6 μm), in which myosin filaments lie, looks dark under a microscope. This dark area can be traced across the entire muscle fiber, since the sarcomeres of neighboring myofibrils are located strictly symmetrically one above the other. The dark areas of sarcomeres are called A-discs from the word "anisotropic". These areas have birefringence in polarized light. The areas at the edges of the A-disk, where actin and myosin filaments overlap, appear darker than in the center, where only myosin filaments are found. This central region is called the H stripe.

The areas of the myofibril, in which only actin filaments are located, do not have birefringence, they are isotropic. Hence their name - I-discs. In the center of the I-disk there is a narrow dark line formed by the Z-membrane. This membrane keeps the actin filaments of two adjacent sarcomeres in an ordered state.

The composition of the actin filament, in addition to actin molecules, also includes the proteins tropomyosin and troponin, which affect the interaction of actin and myosin filaments. In the myosin molecule, there are sections that are called the head, neck and tail. Each such molecule has one tail and two heads with necks. Each head has a chemical center that can attach ATP and a site that allows it to bind to the actin filament.

During the formation of a myosin filament, myosin molecules are intertwined with their long tails located in the center of this filament, and the heads are closer to its ends (Fig. 5.4). The neck and head form a protrusion protruding from the myosin filaments. These projections are called transverse bridges. They are mobile, and thanks to such bridges, myosin filaments can establish a connection with actin filaments.

When ATP is attached to the head of the myosin molecule, the bridge is briefly at an obtuse angle relative to the tail. At the next moment, partial splitting of ATP occurs and due to this, the head rises, goes into an energized position, in which it can bind to the actin filament.

Actin molecules form a double helix Trolonin

Communication center with ATP

Myosin molecule at high magnification

A section of a thick filament (the heads of myosin molecules are visible)

actin filament

Head

+Ca 2+

ADP-F

Rice. 5.4. The structure of actin and myosin filaments, the movement of myosin heads during muscle contraction and relaxation. Explanation in the text: 1-4 - stages of the cycle

Mechanism of muscle fiber contraction. Excitation of a skeletal muscle fiber under physiological conditions is caused only by impulses coming from motor neurons. The nerve impulse activates the neuromuscular synapse, causes the occurrence of PK.P, and the end plate potential provides the generation of an action potential at the sarcolemma.

The action potential propagates both along the surface membrane of the muscle fiber and deep into the transverse tubules. In this case, depolarization of the cisterns of the sarcoplasmic reticulum and the opening of Ca 2+ channels occur. Since the concentration of Ca 2+ in the sarcoplasm is 1 (G 7 -1 (G b M), and in the cisterns it is approximately 10,000 times higher, when the Ca 2+ channels open, calcium leaves the cisterns along the concentration gradient into the sarcoplasm, diffuses to myofilaments and starts processes that ensure contraction.Thus, the release of Ca 2+ ions

into the sarcoplasm is a factor conjugating the electrical skies and mechanical phenomena in the muscle fiber. Ca 2+ ions bind to troponin and this, with the participation of tropomyo- zina, leads to the opening (unblocking) of actin regions howl filaments that can bind to myosin. After that, the energized myosin heads form bridges with actin, and the final breakdown of ATP, previously captured and retained by the myosin heads, occurs. The energy received from the splitting of ATP is used to turn the myosin heads towards the center of the sarcomere. With this rotation, the myosin heads pull the actin filaments along, moving them between the myosin filaments. In one stroke, the head can advance the actin filament by -1% of the sarcomere length. For maximum contraction, repeated rowing movements of the heads are needed. This occurs when there is a sufficient concentration of ATP and Sa 2+ in the sarcoplasm. For the myosin head to move again, a new ATP molecule must be attached to it. The connection of ATP causes a break in the connection between the myosin head and actin, and for a moment it takes its original position, from which it can proceed to interact with a new section of the actin filament and make a new rowing movement.

This theory of the mechanism of muscle contraction is called the theory of "sliding threads"

To relax the muscle fiber, it is necessary that the concentration of Ca 2+ ions in the sarcoplasm become less than 10 -7 M/l. This is due to the functioning of the calcium pump, which overtakes Ca 2+ from the sarcoplasm to the reticulum. In addition, for muscle relaxation, it is necessary that the bridges between the myosin heads and actin are broken. Such a gap occurs in the presence of ATP molecules in the sarcoplasm and their binding to the myosin heads. After the heads are detached, elastic forces stretch the sarcomere and move the actin filaments to their original position. Elastic forces are formed due to: 1) elastic traction of helical cellular proteins included in the structure of the sarcomere; 2) elastic properties of the membranes of the sarcoplasmic reticulum and sarcolemma; 3) the elasticity of the connective tissue of the muscle, tendons and the action of gravitational forces.

Muscle strength. The strength of a muscle is determined by the maximum value of the load that it can lift, or by the maximum force (tension) that it can develop under conditions of isometric contraction.

A single muscle fiber is capable of developing a tension of 100-200 mg. There are approximately 15-30 million fibers in the body. If they acted in parallel in one direction and at the same time, they could create a voltage of 20-30 tons.

Muscle strength depends on a number of morphofunctional, physiological and physical factors.

Muscle strength increases with an increase in their geometric and physiological cross-sectional area. To determine the physiological cross section of a muscle, the sum of the cross sections of all muscle fibers is found along a line drawn perpendicular to the course of each muscle fiber.

In a muscle with a parallel course of fibers (tailoring), the geometric and physiological cross sections are equal. In muscles with an oblique course of fibers (intercostal), the physiological section is larger than the geometric one, and this contributes to an increase in muscle strength. The physiological section and strength of muscles with a feathery arrangement (most of the muscles of the body) of muscle fibers increases even more.

To be able to compare the strength of muscle fibers in muscles with different histological structures, the concept of absolute muscle strength was introduced.

Absolute muscle strength- the maximum force developed by the muscle, in terms of 1 cm 2 of the physiological cross section. The absolute strength of the biceps - 11.9 kg / cm 2, the triceps muscle of the shoulder - 16.8 kg / cm 2, the calf 5.9 kg / cm 2, smooth - 1 kg / cm 2

The strength of a muscle depends on the percentage of different types of motor units that make up that muscle. The ratio of different types of motor units in the same muscle in people is not the same.

The following types of motor units are distinguished: a) slow, tireless (have a red color) - they have little strength, but can be in a state of tonic contraction for a long time without signs of fatigue; b) fast, easily fatiguable (have a white color) - their fibers have a great force of contraction; c) fast, resistant to fatigue - they have a relatively large force of contraction and fatigue slowly develops in them.

In different people, the ratio of the number of slow and fast motor units in the same muscle is genetically determined and can vary significantly. Thus, in the quadriceps muscle of the human thigh, the relative content of copper fibers can vary from 40 to 98%. The greater the percentage of slow fibers in human muscles, the more they are adapted to long-term, but low-power work. Individuals with a high proportion of fast strong motor units are able to develop great strength but are prone to fatigue quickly. However, it must be borne in mind that fatigue also depends on many other factors.

Muscle strength increases with moderate stretching. This is due to the fact that moderate stretching of the sarcomere (up to 2.2 μm) increases the number of bridges that can form between actin and myosin. When a muscle is stretched, elastic traction also develops in it, aimed at shortening. This thrust is added to the force developed by the movement of the myosin heads.

Muscle strength is regulated by the nervous system by changing the frequency of impulses sent to the muscle, synchronizing the excitation of a large number of motor units, and choosing the types of motor units. The strength of contractions increases: a) with an increase in the number of excited motor units involved in the response; b) with an increase in the frequency of excitation waves in each of the activated fibers; c) during synchronization of excitation waves in muscle fibers; d) upon activation of strong (white) motor units.

First (if a small effort is needed), slow, tireless motor units are activated, then fast, fatigue-resistant ones. And if it is necessary to develop a force of more than 20-25% of the maximum, then fast easily fatigued motor units are involved in the contraction.

At a voltage of up to 75% of the maximum possible, almost all motor units are activated and a further increase in strength occurs due to an increase in the frequency of impulses coming to the muscle fibers.

With weak contractions, the frequency of impulses in the axons of motor neurons is 5-10 imp/s, and with a large force of contraction it can reach up to 50 imp/s.

In childhood, the increase in strength is mainly due to an increase in the thickness of muscle fibers, and this is due to an increase in the number of myofibrils. The increase in the number of fibers is insignificant.

When training the muscles of adults, an increase in their strength is associated with an increase in the number of myofibrils, while an increase in endurance is due to an increase in the number of mitochondria and the intensity of ATP synthesis due to aerobic processes.

There is a relationship between strength and speed of shortening. The rate of muscle contraction is the higher, the greater its length (due to the summation of the contractile effects of sarcomeres) and depends on the load on the muscle. As the load increases, the rate of contraction decreases. Heavy loads can only be lifted when moving slowly. The maximum contraction speed achieved during human muscle contraction is about 8 m/s.

The strength of muscle contraction decreases with the development of fatigue.

Fatigue and its physiological basis.fatigue called a temporary decrease in performance, due to previous work and disappearing after a period of rest.

Fatigue is manifested by a decrease in muscle strength, speed and accuracy of movements, a change in the performance of the cardiorespiratory system and autonomic regulation, and a deterioration in the performance of the functions of the central nervous system. The latter is evidenced by a decrease in the speed of the simplest mental reactions, a weakening of attention, memory, a deterioration in the indicators of thinking, and an increase in the number of erroneous actions.

Subjectively, fatigue can be manifested by a feeling of fatigue, the appearance of muscle pain, palpitations, symptoms of shortness of breath, a desire to reduce the load or stop working. Symptoms of fatigue can vary depending on the type of work, its intensity and degree of fatigue. If fatigue is caused by mental work, then, as a rule, symptoms of a decrease in the functional capabilities of mental activity are more pronounced. With very heavy muscular work, symptoms of disorders at the level of the neuromuscular apparatus may come to the fore.

Fatigue, which develops in the conditions of normal labor activity, both during muscular and mental work, has largely similar mechanisms of development. In both cases, the processes of fatigue develop first in the nervous centers. One indicator of this is a decrease in the mind natural working capacity with physical fatigue, and with mental fatigue - a decrease in efficiency we cervical activities.

rest called the state of rest or the performance of a new activity, in which fatigue is eliminated and working capacity is restored. THEM. Sechenov showed that the restoration of working capacity occurs faster if, when resting after fatigue of one muscle group (for example, the left hand), work is performed by another muscle group (the right hand). He called this phenomenon "active recreation"

Recovery called the processes that ensure the elimination of a shortage of energy and plastic substances, the reproduction of structures used up or damaged during operation, the elimination of excess metabolites and deviations of homeostasis from the optimal level.

The duration of the period necessary for the recovery of the body depends on the intensity and duration of the work. The greater the intensity of labor, the shorter the time it takes to do periods of rest.

Various indicators of physiological and biochemical processes are restored at different times from the end of physical activity. One of the important tests of recovery rate is to determine the time during which the heart rate returns to the level characteristic of the rest period. The recovery time for heart rate after a moderate exercise test in a healthy person should not exceed 5 minutes.

With very intense physical activity, fatigue phenomena develop not only in the central nervous system, but also in neuromuscular synapses, as well as muscles. In the system of the neuromuscular preparation, nerve fibers have the least fatigue, the neuromuscular synapse has the greatest fatigue, and the muscle occupies an intermediate position. Nerve fibers can conduct high frequency action potentials for hours without signs of fatigue. With frequent activation of the synapse, the efficiency of excitation transmission first decreases, and then a blockade of its conduction occurs. This is due to a decrease in the supply of the mediator and ATP in the presynaptic terminal, a decrease in the sensitivity of the postsynaptic membrane to acetylcholine.

A number of theories of the mechanism for the development of fatigue in a very intensively working muscle have been proposed: a) the theory of "exhaustion" - the depletion of ATP reserves and sources of its formation (creatine phosphate, glycogen, fatty acids), b) the theory of "suffocation" - the lack of oxygen delivery is put forward in the first place in the fibers of the working muscle; c) the "clogging" theory, which explains fatigue by the accumulation of lactic acid and toxic metabolic products in the muscle. At present, it is believed that all these phenomena take place during very intensive work of the muscle.

It has been established that the maximum physical work before the development of fatigue is performed at an average severity and pace of labor (the rule of average loads). In the prevention of fatigue, the following are also important: the correct ratio of periods of work and rest, the alternation of mental and physical work, accounting for circadian (circadian), annual and individual biological rhythms.

muscle power is equal to the product of muscle strength and the speed of shortening. Maximum power develops at an average speed of muscle shortening. For the arm muscle, the maximum power (200 W) is achieved at a contraction speed of 2.5 m/s.

5.2. Smooth muscles

Physiological properties and features of smooth muscles.

Smooth muscles are an integral part of some internal organs and are involved in providing the functions performed by these organs. In particular, they regulate the patency of the bronchi for air, blood flow in various organs and tissues, the movement of fluids and chyme (in the stomach, intestines, ureters, urinary and gall bladders), expel the fetus from the uterus, dilate or narrow the pupils (due to the reduction of radial or circular muscles of the iris), change the position of the hair and skin relief. Smooth muscle cells are spindle-shaped, 50-400 µm long, 2-10 µm thick.

Smooth muscles, like skeletal muscles, are excitable, conductive, and contractile. Unlike skeletal muscles, which have elasticity, smooth muscles are plastic (able to maintain the length given to them by stretching for a long time without increasing stress). This property is important for the function of depositing food in the stomach or fluids in the gallbladder and bladder.

Peculiarities excitability smooth muscle fibers are to a certain extent associated with their low transmembrane potential (E 0 = 30-70 mV). Many of these fibers are automatic. The duration of the action potential in them can reach tens of milliseconds. This happens because the action potential in these fibers develops mainly due to the entry of calcium into the sarcoplasm from the intercellular fluid through the so-called slow Ca 2+ channels.

Speed excitation in smooth muscle cells small - 2-10 cm / s. Unlike skeletal muscles, excitation in a smooth muscle can be transmitted from one fiber to another nearby. Such a transfer occurs due to the presence of nexuses between smooth muscle fibers, which have low resistance to electric current and ensure the exchange between Ca 2+ cells and other molecules. As a result, smooth muscle has the properties of functional syncytium.

Contractility smooth muscle fibers is characterized by a long latent period (0.25-1.00 s) and a long duration (up to 1 min) of a single contraction. Smooth muscles have a low contraction force, but are able to stay in tonic contraction for a long time without developing fatigue. This is due to the fact that smooth muscle consumes 100-500 times less energy to maintain tetanic contraction than skeletal muscle. Therefore, the ATP reserves consumed by the smooth muscle have time to recover even during contraction, and the smooth muscles of some body structures are in a state of tonic contraction all their lives.

Conditions for smooth muscle contraction. The most important feature of smooth muscle fibers is that they are excited under the influence of numerous stimuli. Normal skeletal muscle contraction is initiated only by a nerve impulse arriving at the neuromuscular synapse. Smooth muscle contraction can be caused by both nerve impulses and biologically active substances (hormones, many neurotransmitters, prostaglandins, some metabolites), as well as physical factors, such as stretching. In addition, smooth muscle excitation can occur spontaneously - due to automaticity.

The very high reactivity of smooth muscles, their ability to respond with contraction to the action of various factors, creates significant difficulties for correcting violations of the tone of these muscles in medical practice. This can be seen in the examples of the treatment of bronchial asthma, arterial hypertension, spastic colitis and other diseases that require correction of the contractile activity of smooth muscles.

The molecular mechanism of smooth muscle contraction also has a number of differences from the mechanism of skeletal muscle contraction. Actin and myosin filaments in smooth muscle fibers are less ordered than in skeletal ones, and therefore smooth muscle does not have transverse striation. There is no troponin protein in actin filaments of smooth muscle, and actin molecular centers are always open for interaction with myosin heads. For this interaction to occur, splitting of ATP molecules and transfer of phosphate to the myosin heads is necessary. Then the myosin molecules intertwine into threads and bind their heads to myosin. This is followed by the rotation of the myosin heads, in which the actin filaments are drawn in between the myosin filaments and contraction occurs.

Phosphorylation of myosin heads is carried out by the enzyme myosin light chain kinase, and dephosphorylation by myosin light chain phosphatase. If the activity of myosin phosphatase predominates over the activity of the kinase, then the myosin heads are dephosphorylated, the connection between myosin and actin is broken, and the muscle relaxes.

Therefore, for smooth muscle contraction to occur, an increase in the activity of myosin light chain kinase is necessary. Its activity is regulated by the level of Ca 2+ in the sarcoplasm. When a smooth muscle fiber is stimulated, the calcium content in its sarcoplasm increases. This increase is due to the intake of Ca^ + from two sources: 1) intercellular space; 2) sarcoplasmic reticulum (Fig. 5.5). Further, Ca 2+ ions form a complex with the calmodulin protein, which activates myosin kinase.

The sequence of processes leading to the development of smooth muscle contraction: the entry of Ca 2 into the sarcoplasm - acti

calmodulin vation (by forming a complex 4Ca 2+ - calmodulin) - activation of myosin light chain kinase - phosphorylation of myosin heads - binding of myosin heads to actin and head rotation, in which the actin filaments are pulled between the myosin filaments.

Conditions necessary for smooth muscle relaxation: 1) reduction (up to 10 M/l or less) of Ca 2+ content in the sarcoplasm; 2) the breakdown of the 4Ca 2+ -calmodulin complex, leading to a decrease in the activity of myosin light chain kinase - dephosphorylation of myosin heads, leading to a break in the bonds of actin and myosin filaments. After that, the elastic forces cause a relatively slow recovery of the original length of the smooth muscle fiber, its relaxation.

Control questions and tasks

cell membrane

Rice. 5.5. Scheme of the pathways of Ca 2+ entry into the sarcoplasm of smooth muscle

of the cell and its removal from the plasma: a - mechanisms that ensure the entry of Ca 2 + into the sarcoplasm and the start of contraction (Ca 2+ comes from the extracellular environment and the sarcoplasmic reticulum); b - ways to remove Ca 2+ from the sarcoplasm and ensure relaxation

Influence of norepinephrine through a-adrenergic receptors

Ligand-dependent Ca 2+ channel

Channels "g leak

Potential dependent Ca 2+ channel

smooth muscle cell

a-adreno! receptorfNorepinephrineG

Name the types of human muscles. What are the functions of skeletal muscles?

Describe the physiological properties of skeletal muscles.

What is the ratio of action potential, contraction and excitability of the muscle fiber?

What are the modes and types of muscle contractions?

Give the structural and functional characteristics of the muscle fiber.

What are motor units? List their types and features.

What is the mechanism of contraction and relaxation of a muscle fiber?

What is muscle strength and what factors affect it?

What is the relationship between the force of contraction, its speed and work?

Define fatigue and recovery. What are their physiological bases?

What are the physiological properties and characteristics of smooth muscles?

List the conditions for contraction and relaxation of smooth muscle.

The structural and functional unit of a skeletal muscle is a symplast or muscle fiber - a huge cell that has the shape of an extended cylinder with pointed edges (the name symplast, muscle fiber, muscle cell should be understood as the same object).

The length of the muscle cell most often corresponds to the length of the whole muscle and reaches 14 cm, and the diameter is equal to several hundredths of a millimeter. skeletal muscle structure development

Muscle fiber, like any cell, is surrounded by a shell - sarcolemma. Outside, individual muscle fibers are surrounded by loose connective tissue, which contains blood and lymphatic vessels, as well as nerve fibers.

Groups of muscle fibers form bundles, which, in turn, are combined into a whole muscle, placed in a dense cover of connective tissue passing at the ends of the muscle into tendons attached to the bone (Fig. 1).

Rice. 1.

The force caused by the contraction of the length of the muscle fiber is transmitted through the tendons to the bones of the skeleton and sets them in motion.

The contractile activity of the muscle is controlled by a large number of motor neurons (Fig. 2) - nerve cells whose bodies lie in the spinal cord, and long branches - axons as part of the motor nerve approach the muscle. Entering the muscle, the axon branches into many branches, each of which is connected to a separate fiber.

Rice. 2.

So one motor neuron innervates a whole group of fibers (the so-called neuromotor unit), which works as a whole.

The muscle consists of many neuromotor units and is able to work not with its entire mass, but in parts, which allows you to regulate the strength and speed of contraction.

To understand the mechanism of muscle contraction, it is necessary to consider the internal structure of the muscle fiber, which, as you already understood, is very different from a normal cell. Let's start with the fact that the muscle fiber is multinucleated. This is due to the peculiarities of fiber formation during the development of the fetus. Symplasts (muscle fibers) are formed at the stage of embryonic development of the organism from precursor cells - myoblasts.

Myoblasts(unformed muscle cells) intensively divide, merge and form muscle tubes with a central arrangement of nuclei. Then, the synthesis of myofibrils begins in the myofibrils (contractile structures of the cell, see below), and the formation of the fiber is completed by the migration of nuclei to the periphery. By this time, the nuclei of the muscle fiber already lose their ability to divide, and only the function of generating information for protein synthesis remains behind them.

But not all myoblasts follow the path of fusion, some of them are isolated in the form of satellite cells located on the surface of the muscle fiber, namely in the sarcolemum, between the plasma membrane and the basement membrane - the constituent parts of the sarcolemum. Satellite cells, unlike muscle fibers, do not lose the ability to divide throughout life, which ensures an increase in the muscle mass of the fibers and their renewal. Recovery of muscle fibers in case of muscle damage is possible due to satellite cells. With the death of the fibers hiding in its shell, satellite cells are activated, divide and transform into myoblasts.

Myoblasts merge with each other and form new muscle fibers, in which the assembly of myofibrils then begins. That is, during regeneration, the events of the embryonic (intrauterine) development of the muscle are completely repeated.

In addition to multinucleation, a distinctive feature of the muscle fiber is the presence in the cytoplasm (in the muscle fiber it is commonly called sarcoplasm) of thin fibers - myofibrils (Fig. 1), located along the cell and laid parallel to each other. The number of myofibrils in the fiber reaches two thousand.

myofibrils are contractile elements of the cell and have the ability to reduce their length when a nerve impulse arrives, thereby tightening the muscle fiber. Under a microscope, it can be seen that the myofibril has a transverse striation - alternating dark and light stripes.

When reducing myofibrils light areas reduce their length and disappear completely with full contraction. To explain the mechanism of myofibril contraction, about fifty years ago, Hugh Huxley developed a model of sliding filaments, then it was confirmed in experiments and is now generally accepted.

The main functional properties of muscle tissue include excitability, contractility, extensibility, elasticity and plasticity.

Excitability - the ability of muscle tissue to come into a state of excitation under the action of certain stimuli. Under normal conditions, electrical excitation of the muscle occurs, caused by the discharge of motor neurons in the region of the end plates. The end plate potential (EPP) arising under the influence of the mediator, having reached the threshold level (about 30 mV), causes the generation of an action potential that propagates in both directions of the muscle fiber.

The excitability of muscle fibers is lower than the excitability of the nerve fibers that innervate the muscles, although the critical level of membrane depolarization is the same in both cases. This is due to the fact that the resting potential of muscle fibers is higher (about 90 mV) than the resting potential of nerve fibers (70 mV). Therefore, for the occurrence of an action potential in the muscle fiber, it is necessary to depolarize the membrane by a greater amount than in the nerve fiber.

The ability of a muscle to respond to irritation of its motor neuron, i.e. to impulses coming to it along the nerve, is designated as indirect excitability of the muscle. However, the muscle fiber itself also has excitability. This is proved by irritation of muscle areas where there are no motor nerve endings.

It is possible to exclude the influence of nerve elements on the muscle by subjecting it to poisoning with certain poisons (for example, curare). In this case, excitation from the nerve to the muscle is not transmitted, but the nerve and muscle continue to function by themselves, i.e. the muscle continues to respond to the stimulation directly applied to it. Thus, experiments of this kind undoubtedly establish the presence in the muscle fiber of the so-called direct excitability, i.e. the ability of muscle fibers to respond to irritation that acts directly on them, and not through nerve fibers.

Both direct and indirect muscle excitability is due to the function of the muscle fiber membrane. Excitation in the muscles is carried out in isolation, i.e. does not move from one muscle fiber to another. The speed of propagation of excitation in the white and red fibers of skeletal muscles is different: in white fibers it is 12 - 15, in red - 3 - 4 m / s.

Muscles have a passive elastic component, which includes tendons, connective tissue covering muscle fibers, their bundles and the muscle as a whole, as well as elastic formations of lateral transverse bridges of the myosin filament. Therefore, skeletal muscle is an elastic formation. Elasticity is possessed by active contractile and passive components of the muscle, which provide extensibility, elasticity and plasticity of the muscles.

Extensibility - the property of a muscle to lengthen under the influence of gravity (load). The greater the load, the greater the extensibility of the muscle. Extensibility also depends on the type of muscle fibers. Red fibers stretch more than white, parallel fibers stretch more than cirrus. Even at rest, the muscles are always somewhat stretched, so they are elastically tense (they are in a state of muscle tone).

Elasticity - the property of a deformed body to return to its original state after the removal of the force that caused the deformation. This property is studied when the muscle is stretched with a load. After removal of the load, the muscle does not always reach its original length, especially with prolonged stretching or under the influence of a large load. This is due to the fact that the muscle loses the property of perfect elasticity.

Plasticity - (Greek Plastikos - suitable for sculpting, pliable) the property of a body to deform under the action of mechanical loads, to retain the given length or shape after the termination of the external deforming force. The longer a large external force acts, the stronger the plastic changes.

Muscle plasticity is also associated with residual shortening of muscles after prolonged tetanic contraction, or contracture. Red fibers, which hold the body in a certain position, have greater plasticity than white ones.

With direct or indirect stimulation, the muscle shortens or develops tension in the longitudinal direction. This change in the shape or tension of the muscle is called muscle contraction, therefore, contractility is the specific activity of muscle tissue when it is excited.

To study the properties of muscles for educational purposes and in the experiment, a neuromuscular preparation of a frog is usually used as an object, and an electric current is used as an irritant. Recording muscle contractions on a myograph device with direct or indirect stimulation is called myography. The speed and strength of the skeletal muscle response to irritation depends not only on the parameters of the stimulus, but also on the type of muscle fibers. The contractility and excitability of muscles of different types is different.

According to the speed of contraction, fast and slow muscle fibers are distinguished. In fast fibers, the sarcoplasmic reticulum is usually better developed, they are less supplied with blood vessels, they have larger and longer fibers, their relaxation after contraction occurs 50-100 times faster than slow fibers. The body uses mainly slow, tonic red muscles to perform static work (for example, maintaining a posture), and fast white muscles for high-speed movements.

There are different modes of muscle contraction, which are determined by the frequency and strength of the incoming excitation impulses.

To direct and indirect stimuli with a frequency of not more than 6-8 Hz, the muscle, consisting of slow motor units, responds with single contractions. The contraction does not occur immediately after the application of irritation, but after a certain period of time, called the latent period. Its value is 0.01 s for the frog gastrocnemius muscle. The shortening phase lasts 0.04 s, the relaxation phase lasts 0.05 s.

The onset of the contraction corresponds to the ascending phase of the action potential when it reaches the threshold value (approximately 40 mV). In mammals, a single contraction of skeletal muscles lasts 0.04 - 0.1 s, but it is not the same in different muscles in the same animal. In red muscle fibers, it is much greater than in white. If a muscle is acted upon by two stimuli that quickly follow each other (the period between impulses is no more than 100 ms), the muscle fibers do not fully relax and each subsequent contraction seems to be layered on the previous one. There is a summation of contractions, which can be complete, when both contractions merge, forming one peak, or incomplete, depending on the frequency of stimulation. In both cases, the contraction has a greater amplitude than the maximum contraction with a single stimulation.

When a muscle is exposed to rhythmic stimuli of high frequency, a strong and prolonged contraction of the muscle occurs, which is called tetanic contraction or tetanus. This term was first used by E. Weber in 1821.

Tetanus can be serrated (at a frequency of stimuli of 20-40 Hz) or continuous, smooth (at a frequency of 50 Hz and above). The amplitude of tetanic contraction is 2-4 times higher than the amplitude of a single contraction with the same strength of stimulation.

Smooth tetanus occurs when the next impulse of irritation acts on the muscle before the start of the relaxation phase. With a very high frequency of stimulation, each subsequent irritation will fall into the phase of absolute refractoriness and the muscle will not contract at all. The height of muscle contraction in tetanus depends on the rhythm of stimulation, as well as on excitability and lability, which change during muscle contraction. Tetanus is highest at the optimal rhythm, when each subsequent impulse acts on the muscle in the exaltation phase caused by the previous impulse. In this case, the best conditions are created (optimum strength and frequency of stimulation, optimum rhythm) for muscle work.

With tetanic contractions, muscle fibers tire more than with single contractions. Therefore, even within the same muscle there is a periodic change in the frequency of impulses (up to complete disappearance) in different motor units.

Impulses from motor neurons at rest are involved in maintaining muscle tone.

Under the tone understand the state of natural constant muscle tension at low energy costs. Muscle proprioceptors (muscle spindles) and the central nervous system are involved in maintaining tone.

The implementation of skeletal muscle tone is due to the function of slow motor units of red muscle fibers. The tone of skeletal muscles is associated with the flow of rare nerve impulses to the muscle, as a result of which the muscle fibers are excited not simultaneously, but alternately. Domestic animals have specialized reflex arcs, some of which provide tetanic contractions, while others provide muscle tone. Skeletal muscle tone plays an important role in maintaining a certain position of the body in space and the activity of the motor apparatus.

When the actin and myosin fibrils approach each other, due to the closing of the transverse bridges, tension develops in the muscle fiber (active mechanical traction). Depending on the conditions in which muscle contraction occurs, the developing tension is realized in different ways. There are two main types of muscle contractions - isotonic and isometric. When a muscle contracts during irritation without lifting any load, the muscle fibers shorten, but their tension does not change and is equal to zero, such a contraction is called isotonic (Greek isos - equal, tonos - tension). In the experiment, isotonic contraction is obtained by electrical (tetanic) stimulation of an isolated muscle weighed down by a small load. Shortening of the muscle occurs at a constant voltage equal to the external load.

Isometric (Greek isos - equal, meros - measure) is a contraction in which the length of the fibers does not decrease, but their tension increases (reduction with a constant length). In this case, the contractile component is shortened due to stretching of the passive elastic component, which can increase its length by 2–6% of the resting length.

From a molecular point of view, tension during isotonic contraction is provided by the closing and opening of transverse bridges. In this case, the rate of contraction depends on the number of closed bridges formed per unit time (the fewer of them, the greater the speed and, accordingly, the less force of contraction).

During isometric contraction, tension in the muscle fibers is created due to the reattachment of transverse bridges on the same fixed sections of actin filaments.

Under natural conditions of muscle activity, there is practically no purely isotonic or purely isometric contraction.

A mixed type of muscle contraction, in which length and tension change, is called auxotonic. When an animal performs complex motor acts, all working muscles contract auxotonically - with a predominance of either isotonic or isometric type of contraction.

Structural and functional unit skeletal muscle is symplast or muscle fiber- a huge cell that has the shape of an extended cylinder with pointed edges (under the name symplast, muscle fiber, muscle cell should be understood as the same object).

The length of the muscle cell most often corresponds to the length of the whole muscle and reaches 14 cm, and the diameter is equal to several hundredths of a millimeter.

muscle fiber, like any cell, is surrounded by a shell - a sarcolemma. Outside, individual muscle fibers are surrounded by loose connective tissue, which contains blood and lymphatic vessels, as well as nerve fibers.

Groups of muscle fibers form bundles, which, in turn, are combined into a whole muscle, placed in a dense cover of connective tissue passing at the ends of the muscle into tendons attached to the bone (Fig. 1).

Rice. 1.

The force caused by the contraction of the length of the muscle fiber is transmitted through the tendons to the bones of the skeleton and sets them in motion.

The contractile activity of the muscle is controlled by a large number of motor neurons (Fig. 2) - nerve cells whose bodies lie in the spinal cord, and long branches - axons as part of the motor nerve approach the muscle. Entering the muscle, the axon branches into many branches, each of which is connected to a separate fiber.

Rice. 2.

So one motor neuron innervates a whole group of fibers (the so-called neuromotor unit), which works as a whole.

The muscle consists of many neuromotor units and is able to work not with its entire mass, but in parts, which allows you to regulate the strength and speed of contraction.

To understand the mechanism of muscle contraction, it is necessary to consider the internal structure of the muscle fiber, which, as you already understood, is very different from a normal cell. Let's start with the fact that the muscle fiber is multinucleated. This is due to the peculiarities of fiber formation during the development of the fetus. Symplasts (muscle fibers) are formed at the stage of embryonic development of the organism from precursor cells - myoblasts.

Myoblasts(unformed muscle cells) intensively divide, merge and form muscle tubes with a central arrangement of nuclei. Then, the synthesis of myofibrils begins in the myofibrils (contractile structures of the cell, see below), and the formation of the fiber is completed by the migration of nuclei to the periphery. By this time, the nuclei of the muscle fiber already lose their ability to divide, and only the function of generating information for protein synthesis remains behind them.

But not all myoblasts follow the path of fusion, some of them are isolated in the form of satellite cells located on the surface of the muscle fiber, namely in the sarcolemum, between the plasma membrane and the basement membrane - the constituent parts of the sarcolemum. Satellite cells, unlike muscle fibers, do not lose the ability to divide throughout life, which ensures an increase in the muscle mass of the fibers and their renewal. Recovery of muscle fibers in case of muscle damage is possible due to satellite cells. With the death of the fibers hiding in its shell, satellite cells are activated, divide and transform into myoblasts.

Myoblasts merge with each other and form new muscle fibers, in which the assembly of myofibrils then begins. That is, during regeneration, the events of the embryonic (intrauterine) development of the muscle are completely repeated.

In addition to multinucleation, a distinctive feature of the muscle fiber is the presence in the cytoplasm (in the muscle fiber it is commonly called sarcoplasm) of thin fibers - myofibrils (Fig. 1), located along the cell and laid parallel to each other. The number of myofibrils in the fiber reaches two thousand.

myofibrils are contractile elements of the cell and have the ability to reduce their length when a nerve impulse arrives, thereby tightening the muscle fiber. Under a microscope, it can be seen that the myofibril has a transverse striation - alternating dark and light stripes.

When reducing myofibrils light areas reduce their length and disappear completely with full contraction. To explain the mechanism of myofibril contraction, about fifty years ago, Hugh Huxley developed a model of sliding filaments, then it was confirmed in experiments and is now generally accepted.

LITERATURE

1. McRobert S. Hands of a titan. – M.: SP "Weider sport", 1999.
2. Ostapenko L. Overtraining. Causes of overtraining during strength training // Ironman, 2000, No. 10-11.
3. Solodkov A.S., Sologub E.B. Physiology of sports: Textbook. - St. Petersburg: SPbGAFK im. P.F. Lesgaft, 1999.
4. Physiology of muscular activity: Textbook for institutes of physical culture / Ed. Kotsa Ya. M. - M .: Physical culture and sport, 1982.
5. Human Physiology (Textbook for Institutes of Physical Culture. 5th ed.). / Ed. N. V. Zimkina. - M .: Physical culture and sport, 1975.
6. Human Physiology: A Textbook for Medical Students / Ed. Kositsky G.I. - M.: Medicine, 1985.
7. Physiological bases of sports training: Guidelines for sports physiology. - L .: GDOIFK im. P.F. Lesgaft, 1986.